1. Field of the Invention
This invention pertains to the field of wireless digital communication systems, and more particularly, to a method and system for transmitting user signals having various chip rates over carrier frequency band by assigning a channelization code to each user signal.
2. Description of the Related Art
Generally a multiple access communication system transmits or receives information sequences of many users over a same frequency band. Particularly, in a wireless digital communication system using a Code Division Multiple Access (CDMA), a multiple number of users share a common bandwidth of W Hz carrier frequency.
In a CDMA system, a unique binary spreading sequence (a code) is assigned to each user signals of each call. Multiplied by an assigned channelization code, a user signal is “spread” onto a channel bandwidth which is much wider than the user signal bandwidth. The transmitting rate of the assigned channelization code is commonly called “the chip rate” and the chip rate is higher than the bit rate, which is the transmitting rate of the user data.
All active users share the same system bandwidth frequency spectrum at the same time. Thus, the signal of each user is separated from others at the receiver using a correlator matched with the associated channelization code to “de-spread” desired signals. The de-spread signal is then integrated during a chip period.
The ratio of the chip rate to the user bit rate is commonly called the “spreading factor” (SF). The spreading factor represents the length of the assigned channelization code.
While the receiver multiplies the associated channelization code sequence to the user signal for the despreading, the components of other user signals may still remain as noises due to interference or error. These noise can significantly be diminished after passing de-spread signal through an integrator of the receiver.
Thus, if each user signal is synchronized to others and the chip rate of every user signal is same, the interference between user signal data in a CDMA communication system can be removed by allocating code sequences for all users which are orthogonal to each another.
Accordingly, an Orthogonal Variable Spreading Factor (OVSF) code can be applied to the CDMA system. The OVSF code allows the SF of each user signal and the transmitting rate of the user data, i.e. the bit rate, to be different from one another.
FIG. 1 shows a spreader in a transmitter of a CDMA communication system in the related art. Referring to FIG. 1, bk(t) is the data signal of user k and ak(t) is a spreading signal of user k. The data signal bk(t) of user k is spread by being multiplied with the spreading signal ak(t), and a spread signal of s(t) of all users are transmitted through a same system channel bandwidth frequency spectrum. The spread signal s(t) of all users can be expressed with the following equation.                                           s            ⁡                          (              t              )                                =                                    ∑                              l                =                1                            K                        ⁢                                                                                a                    k                                    ⁡                                      (                    t                    )                                                  ·                                                      b                    k                                    ⁡                                      (                    t                    )                                                  ·                cos                            ⁢                                                           ⁢                              ω                c                            ⁢              t                                      ,                            [                  Equation          ⁢                                           ⁢          1                ]            where K is the number of total users, and ωc is the carrier frequency.
The ak(t) and bk(t) can be expressed as follows.                                                         a              k                        ⁡                          (              t              )                                =                                    ∑                              j                =                                  -                  ∞                                            ∞                        ⁢                                          a                                  k                  ,                  j                                            ⁢                              Ψ                ⁡                                  (                                      t                    -                                          j                      ⁢                                                                                           ⁢                                              T                        c                                                                              )                                                                    ,                            [                  Equation          ⁢                                           ⁢          2                ]            where ak,j is the jth chip of the channelization code allocated to the kth user which has alternative value of 1 or −1, Ψ(t) is an impulse of the pulse shaping filter, t is an intermediate variable for time, and Tc is the chip period which is a reciprocal of the chip rate. The ak,j has a period of SFk (i.e. ak,j=ak,j+SFk where k=1, 2, . . . , K). The chip period of all user data is assumed to be the same in the CDMA system of the related art. Also, assuming that the duration of pulse shaping function is equal to chip duration (i.e. Ψ(t)=0 for t<0 or t≧Tc).                                                         b              k                        ⁡                          (              t              )                                =                                    ∑                              m                =                                  -                  ∞                                            ∞                        ⁢                                          b                                  k                  ,                  m                                            ⁢                                                P                  k                                ⁡                                  (                                      t                    -                                          m                      ⁢                                                                                           ⁢                                              T                        k                                                                              )                                                                    ,                            [                  Equation          ⁢                                           ⁢          3                ]            where bk,m is the mth data bit of the kth user which has alternative value of 1 or −1, Tk is the data bit duration of the kth user which is a reciprocal of data bit, and Pk(t) is the rectangular pulse function which is 1 for 0≦t<Tk and 0 otherwise. It is assumed that all users are time-synchronized in order to preserve the orthogonality of OVSF codes.
FIG. 2 shows a de-spreader in a receiver of a conventional CDMA communication system. Referring to FIG. 2, the input signal of the receiver r(t) can be expressed with the following equation.                                           r            ⁡                          (              t              )                                =                                                    A                ⁡                                  (                  t                  )                                            ⁢                              s                ⁡                                  (                                      t                    -                    τ                                    )                                                      +                          n              ⁡                              (                t                )                                                    ,                            [                  Equation          ⁢                                           ⁢          4                ]            where A(t) is a distortion caused when the user signal passes a physical channel, τ is the propagation time delay, and n(t) is the additive white Gaussian noise.
The input signal r(t) of the receiver is multiplied by the desired user spreading signal or code ai(t) and integrated for a data bit duration, where i is the desired user index. If so, the other user signals except the desired user signal are eliminated and the user data signal can be extracted.
For simplification, assume the parameter of distortion equals 1 (A(t)=1), a zero time delay (τ=0), and the noise equals zero (n(t)=0). Then, the nth output Zi(nTi) of the receiver for user i can be expressed as follows.                                                         Z              i                        ⁡                          (                              n                ⁢                                                                   ⁢                                  T                  i                                            )                                =                                    b                              i                ,                n                                      +                                          I                                  k                  ,                  i                                            ⁡                              (                                  n                  ⁢                                                                           ⁢                                      T                    i                                                  )                                                    ,                            [                  Equation          ⁢                                           ⁢          5                ]            where the interference Ik,i(nTi) is the component due to the kth user signal and can be expressed as follows.                                                         I                              k                ,                i                                      ⁡                          (                              n                ⁢                                                                   ⁢                                  T                  i                                            )                                =                                    1                              T                i                                      ·                                          ∫                0                                  T                  0                                            ⁢                                                                    ψ                    2                                    ⁡                                      (                    t                    )                                                  ⁢                                                      ⅆ                    t                                    ·                                                            ∑                                              m                        =                                                  α                                                      k                            ,                            i                                                                                                                      β                                                  k                          ,                          j                                                                                      ⁢                                                                  b                                                  k                          ,                          m                                                                    ⁢                                                                        R                                                      k                            ,                            i                                                                          ⁡                                                  (                          m                          )                                                                                                                                                        ,                            [                  Equation          ⁢                                           ⁢          6                ]            whereαk,i=└nTi/Tk┘      β          k      ,      i        =      {                                                                                        ⌊                                      n                    ⁢                                                                                   ⁢                                                                  T                        i                                            /                                              T                        k                                                                              ⌋                                ,                                                                                      T                  i                                ≤                                  T                  k                                                                                                                                              ⌊                                                                  (                                                                              n                            ⁢                                                                                                                   ⁢                                                          T                              i                                                                                +                                                      T                            i                                                                          )                                            /                                              T                        k                                                              ⌋                                    -                  1                                ,                                                                                      T                  i                                >                                  T                  k                                                                    ⁢                                              ⁢                                  ⁢                              R                          k              ,              i                                ⁡                      (            m            )                              =              {                                                                                                  ∑                                          r                      =                                              n                        ⁢                                                                                                   ⁢                                                  N                          i                                                                                                                                                              (                                                      n                            +                            1                                                    )                                                ⁢                                                  N                          i                                                                    -                      1                                                        ⁢                                                            a                                              k                        ,                        r                                                              ⁢                                          a                                              i                        ,                        r                                                                                            ,                                                                                      T                  i                                ≤                                  T                  k                                                                                                                                              ∑                                          r                      =                                              m                        ⁢                                                                                                   ⁢                                                  N                          k                                                                                                                                                              (                                                      m                            +                            1                                                    )                                                ⁢                                                  N                          i                                                                    -                      1                                                        ⁢                                                            a                                              k                        ,                        r                                                              ⁢                                          a                                              i                        ,                        r                                                                                            ,                                                                                      T                  i                                >                                  T                  k                                                                        in which └x┘ is the maximum integer which is equal to or less than x.
To obtain the Interference signal Ik,i(nTi), the value is held to be zero regardless of the transmitted user data bit bk,m, and the value of Rk,i(m), m=αk,i, . . . βk,i should be equal to zero. The code that satisfies this condition is the OVSF code.
The OVSF code of the condition, Cch,SF,n, n=0, 2, . . . , SF−1 is shown as a code tree in FIG. 3. Such code tree is disclosed in 3GPP RAN 25.213, V3.0.2 (March 2000), Spreading and modulation (FDD) and is fully incorporated herein. The Cch,1,0=(1) is the beginning of code tree, the two branch code Cch,2,0 and Cch,2,1 are divided out from the trunk code Cch,1,0. That is, the code Cch,2,0 is made by repeating code Cch,1,0, and Cch,2,1 is made by connecting two codes of code Cch,1,0 and a code made by multiplying code Cch,1,0 with (−1). The branches of the code tree can be extended by making two branches from each branch of the code tree such as code Cch,2,0 and code Cch,2,1 respectively with the same procedure.
The OVSF code can be generated as follows by using a matrix.                                           C                          ch              ,              1              ,              0                                =          1                ,                                  ⁢                              [                                                                                C                                          ch                      ,                      2                      ,                      0                                                                                                                                        C                                          ch                      ,                      2                      ,                      1                                                                                            ]                    =                                    [                                                                                          C                                              ch                        ,                        1                        ,                        0                                                                                                                        C                                              ch                        ,                        1                        ,                        0                                                                                                                                                        C                                              ch                        ,                        1                        ,                        0                                                                                                                        -                                              C                                                  ch                          ,                          1                          ,                          0                                                                                                                                ]                        =                                                            [                                                                                    1                                                                    1                                                                                                            1                                                                                              -                          1                                                                                                      ]                                ⁢                                                                  [                                                                                                    C                                                  ch                          ,                                                      2                                                          (                                                              n                                +                                1                                                            )                                                                                ,                          0                                                                                                                                                                        C                                                  ch                          ,                                                      2                                                          (                                                              n                                +                                1                                                            )                                                                                ,                          1                                                                                                                                                                        C                                                  ch                          ,                                                      2                                                          (                                                              n                                +                                1                                                            )                                                                                ,                          2                                                                                                                                                                        C                                                  ch                          ,                                                      2                                                          (                                                              n                                +                                1                                                            )                                                                                ,                          3                                                                                                                                                ⋮                                                                                                                          C                                                  ch                          ,                                                      2                                                          (                                                              n                                +                                1                                                            )                                                                                ,                                                      2                                                          (                                                              n                                +                                1                                                            )                                                                                ,                          2                                                                                                                                                                        C                                                  ch                          ,                                                      2                                                          (                                                              n                                +                                1                                                            )                                                                                ,                                                      2                                                          (                                                              n                                +                                1                                                            )                                                                                ,                          1                                                                                                                    ]                            =                              [                                                                                                    C                                                  ch                          ,                                                      2                            n                                                    ,                          0                                                                                                                                    C                                                  ch                          ,                                                      2                            n                                                    ,                          0                                                                                                                                                                        C                                                  ch                          ,                                                      2                            n                                                    ,                          0                                                                                                                                    -                                                  C                                                      ch                            ,                                                          2                              n                                                        ,                            0                                                                                                                                                                                                  C                                                  ch                          ,                                                      2                            n                                                    ,                          1                                                                                                                                    C                                                  ch                          ,                                                      2                            n                                                    ,                          1                                                                                                                                                                        C                                                  ch                          ,                                                      2                            n                                                    ,                          1                                                                                                                                    -                                                  C                                                      ch                            ,                                                          2                              n                                                        ,                            1                                                                                                                                                                          ⋮                                                              ⋮                                                                                                                          C                                                  ch                          ,                                                      2                            n                                                    ,                                                                                    2                              n                                                        -                            1                                                                                                                                                              C                                                  ch                          ,                                                      2                            n                                                    ,                                                                                    2                              n                                                        -                            1                                                                                                                                                                                                  C                                                  ch                          ,                                                      2                            n                                                    ,                                                                                    2                              n                                                        -                            1                                                                                                                                                              -                                                  C                                                      ch                            ,                                                          2                              n                                                        ,                                                                                          2                                n                                                            -                              1                                                                                                                                                                          ]                                                                        [                  Equation          ⁢                                           ⁢          7                ]            
The procedure to allocate OVSF code to a user channel is accomplished by selecting a code from codes of spreading factor (SF) which is necessary to the user channel, avoiding three types of codes as follows from the code tree. The types of codes to avoid are as follows. First, the codes that are allocated by other channels should be avoided. Second, in the code tree, all descendant codes of the codes used by other users should be avoided. Third, in the code tree, all ancestor codes of the codes used by other users should be avoided.
In systems of the related art, all user chip rates should be the same and all users should be time-synchronized in order to preserve the orthogonality of OVSF codes, as discussed above. However, in practice, the duration of pulse shaping function is longer than the chip rate.
FIG. 4 is block diagram of a transmitter for single chip rate in a Wideband CDMA (WCDMA) communication system in the related. Referring to FIG. 4, the WCDMA communication system in the related art comprises a serial to parallel (S/P) converter (400) which outputs I signal and Q signal from data or control information; first mixer (401) and second mixer (402) which spread data symbol by multiplying I signal and Q signal output from said serial to parallel converter (400) with channelization code; an imaginary number converter (403) which converts the output of said second mixer (402) into an imaginary number; a combining unit (404) outputting complex number signal by combining I signal from said first mixer (401) and Q signal from said imaginary number converter (403); a third mixer (405) scrambling the complex number signal by multiplying scrambling code; a separating unit (406) separating the scrambled complex number signal into real component and imaginary component; first pulse shaping filter (407) and second pulse shaping filter (408) generating chip waveform in order to transmit the separated output signal through a dedicated frequency band; a fourth mixer (409) loading the output signal of said first pulse shaping filter (407) to the carrier wave by multiplying cos(ωct); and a fifth mixer (410) loading the output signal of said second pulse shaping filter (408) to the carrier wave by multiplying −sin(ωct).
The function of a WCDMA communication system in FIG. 4 is as follows. At first, a user data and control information are spread by channelization code (Cch,SF,n) after separation into I signal and Q signal. After that, the I signal and Q signal are combined to complex number signal, and the combined complex number signal is scrambled by complex-valued scrambling code (Cscramb1). At this time, the OVSF code is used as a channelization code, which discriminates the channel dedicated to each user. The complex-valued scrambling code can be used to discriminate the transmitter as pseudo-random code.
The complex number scrambled at the third mixer (405) is separated into real number component signal and imaginary number component signal at splitter (406), respectively. After, the real number component signal is fed to the pulse shaping filter (407) and modulated with carrier frequency oc (409), the imaginary number component signal is also fed to the pulse shaping filter (408) and modulated with carrier frequency ωc (410).
At this time, the rate of the channelization code and the complex-valued scrambling code in FIG. 4 is 3.84 Mcps, and each pulse shaping filter (407, 408) generates chip waveform in order to transmit the signal of 3.84 Mcps chip rate through 5 MHz frequency band.
As explained above, the channel signals, spread with channelization code, do not cause inter-signal interference because they retain the property of orthogonal to one another. In these WCDMA systems, there is a continuing need to increase the performance of the system by accommodating users having different source rates.
Because the transmitter of the WCDMA communication system in the related art is designed to transmit only single chip rate, that is 3.84 Mcps signal, it has the problem that it cannot transmit other signal which has multiple chip rates of said single chip rate, namely 7.68 Mcps (double of the single chip rate) and 15.36 Mcps (four times of the single chip rate).
In order to solve this problem, a transmitter of a WCDMA communication system in the related art can be expanded to support transmitting multiple chip rates. FIG. 5 shows a transmitter which is capable of transmitting two chip rates. It can be extended to support more than two chip rates. Since the chip rate in WCDMA system is one of 3.84, 7.68, and 15.36 Mcps, the second chip rate is double or four times of the first chip rate. If the first chip rate is 3.84 Mcps, then the second chip rate can be 7.68 Mcps or 15.36 Mcps. If the first chip rate is 7.68 Mcps, then the second chip rate is 15.36 Mcps.
At this time, each chip rate signal is transmitted after being multiplied by both the channelization code and the complex scrambling code, and passing through a dedicated pulse shaping filter. The OVSF codes are allocated as channelization codes under same procedure described with reference to FIG. 3. Particularly, the channel codes are allocated independently to other chip rate group.
The OVSF code can be used in a system of single chip rate as explained above. However, in consideration of multiple chip rate signals in a carrier frequency band, the method to allocate OVSF code to multiple code rate has not been introduced. Therefore, if the OVSF codes are allocated irregularly to the various signals having different chip rate under the procedure in the related art for generating and allocating the OVSF code, the interference between signal to signal occurs because the codes may not be orthogonal to one another.
If the transmitting device supporting single chip rate is expanded to support multiple chip rate, the interference between signal to signal does not occur among signals of same chip rate because of orthogonal property, but the interference between signal to signal among signals of different chip rates would increase, because the scrambling codes are different from one another and the orthogonal property cannot be sustained between channelization codes.